Sponge diversity

Sponges are exclusively sessile and colonial anmals that occur in all aquatic environments with the exception of Antarctic freshwater habitats at high Southern latitudes. Currently, 8,613 sponge species are described. Sponges comprise four classes: Homoscleromorpha, Demospongiae, Hexactinellida and Calcarea. They are suspension feeders with unique morphology, comprising an internal system of water-bearing canals and chamber of ciliated cells. Whilst pumping, these cells remove oxygen and particulate organic matter from the surrounding water and pump out excretion and reproduction products. Sponges have a basic fibrous skeletal system (which consists of spongin fibres often cored by calcareous or siliceous spicules of great diversity in size and shape, Fig. A). The morphological classification of sponges is primarily based on spicule and skeletal morphology. Additionally, advances in molecular techniques provide further insights into species differentiation and higher order classifications.

Sponges have numerous functional roles within their habitats and hence they are important for other organisms, e.g. associations with other organisms, production and release of secondary metabolites. Associations of sponges and other organisms are numerous and sometimes complex. Sponges form microhabitats for smaller sponge-associated organisms (mainly bacteria, algae and smaller invertebrates) which use their hosts as a substrate, food source, shelter or nursery. Crinoids and brittle stars settle on top or stalks of sponges to obtain an exposed position in the bottom water currents and thus increase their nutrient availability. Secondary metabolites are often used as chemical defense against biofouling and predators. These bioactive compounds are also interesting for pharmaceutical and further economical reasons as they might show antimicrobial, -viral, -algal, -fungal, -fouling and inflammatory effects.

In the oligotrophic deep sea, species numbers are lower compared to shelf areas and the composition of sponge communities changes from demosponge to hexactinellid dominated sponge grounds. The morphology is clearly adapted to the deep sea as morphological features like stalks and root tufts are more common. Carnivorous sponges of the demosponge family Cladorhizidae are also known to occur in the deep sea. One carnivorous sponge, maybe of the genus Abyssocladia, was already sampled during the current SO239 EcoResponse cruise (Fig. B). Stalks of deep-sea hexactinellids (even of dead specimens) are important hard substrata and can potentially function like stepping-stones regarding to species distribution of deep-sea fauna, for instance stalks of the genus Hyalonema which was also sampled during several ROV dives in the course of this expedition (Fig. C).